1. Field of the Invention
This invention relates to an alternating current (hereinafter referred to as "AC") voltage regulator, and more particularly to an AC voltage regulator which permits generation of a stable output voltage free from abnormal oscillation components such as almost periodic oscillations and low frequency oscillations.
2. Description of the Prior Art
For communication and data processing systems and instrumentation controlling systems, it is important that their power sources should be maintained at substantially constant voltages. To meet the requirement, numerous voltage regulators of varying principles have been developed and adopted for actual use.
FIG. 2 is a block diagram illustrating one conventional AC voltage regulator.
A resonant capacitor 3 and a reactor 2 are connected in series to an input (commercial) power source 1. Preferably, the reactor 2 and the capacitor 3 have values such as to be in the state of series resonance relative to the power source frequency. A load 10 is connected in parallel with the capacitor 3. A series circuit interconnecting a linear reactor 4 and a switching circuit 7 (such as, for example, a triac or two thyristors bidirectionally connected in parallel) is connected in parallel with the resonant capacitor 3. An output voltage sensing and regulating device 9 is connected in parallel with the load 10 and provides the switching element 7 with an ON-OFF control signal depending on the output (load) voltage.
To be specific, the equivalent reactance of the linear reactor 4 is variably regulated by regulating the firing angle of the switching circuit 7 in accordance with the output signal from the output voltage sensing and regulating device 9.
More specifically, this variable regulation is effected by comparing the load voltage E.sub.0 with the target value and, when the load voltage is higher than the target value, the firing phase angle is advanced according to the difference of the load voltage from the target value so as to increase the current flowing to the linear reactor 4 and lower the output voltage E.sub.0 being applied to the load 10. When the load voltage E.sub.0 is lower than the target value, the variable regulation is effected in the reverse manner.
The constant voltage power source system of FIG. 2 has been finding rapidly growing utility in practical applications because it is held in high esteem for various advantages such as absence of dependency on frequency, less distortion of waveform, and high operational efficiency.
Systems illustrated in FIG. 3 and FIG. 4 which are based on the same operating principle as the AC voltage regulator of FIG. 2 have also been known to the art.
In the system of FIG. 3, the power source side and the load side are interconnected through the medium of a transformer 11 and, in the place of the tuning capacitor 3 of FIG. 2, tuning circuits C3, L3 and C5, L5 for the third harmonic component and the fifth harmonic component are interconnected.
In the system of FIG. 4, the power source side and the load side are interconnected through the medium of a transformer 12 provided with a magnetic shunt and the linear reactor 2 of FIG. 2 is omitted.
Since the circuits for these systems are basically similar to the circuit of the system of FIG. 2, any further description of these circuits is omitted herein.
Since the various systems based on the conventional technique described above inevitably have nonlinearities in their respective circuits, their output voltages can be expected to contain high-frequency oscillation components other than the power source frequency. To be specific, when the equivalent mean inductance of the linear reactor 4 is regulated by on-off controlling the current flowing through the linear reactor 4 by the switching element 7, the current through the linear reactor 4 is caused to assume a distorted waveform to give rise to high frequency components. Further those high frequency components are subject to variation in magnitude due to voltage regulation.
When the load current is large, such high frequency oscillation is repressed by the losses in the load and consequently converted into a feeble oscillation to be synchronized with the power source (fundamental) frequency. Thus, the high frequency oscillation is prevented from manifesting itself in the output voltage. When the load current is particularly light, the high frequency oscillation cannot be synchronized and so oscillations of various frequency components arise and the resultant beat oscillations interfere with one another in a complicated manner and manifest themselves in the output voltage as abnormal oscillations such as almost periodic oscillations or low frequency oscillations.
This phenomenon is the gravest drawback in implementation of a voltage regulator. For prevention of this phenomenon, when the load current is low, the practice of putting a dummy resistance across the load and consequently suppressing the adverse effect of an extremely light load current mentioned above is resorted to.
In this case, the dummy load inevitably, as a result, entails an excess loss and lowers the overall efficiency of the system as a whole. Moreover, since the dummy load entails generation of heat, the system must be provided with a large radiator for release of the heat from the system. Thus, this practice has the disadvantage that the system becomes large and expensive.